Asthma is a disease affecting 100-150 million people worldwide with deaths from asthma reaching 180 000 annually [i]. Asthma affects all age groups but often starts in childhood and is the most common chronic childhood disease affecting 6.3 million children worldwide [ii]. Asthma is also more prevalent in the developing world with the incidence in children increasing by approximately 75% every 10 years in the United States [iii]. However currently there is no easy accepted non-invasive method of measuring pulmonary function in children under the age of 6.
The standard measure of lung function used in older children and adults is spirometry, a learned manoeuvre that depends on the active cooperation of the subject, therefore it does not produce reliable and reproducible results in young children (<6 yr). Techniques used in infants are not suitable over the age of one year, and usually require sedation [iv,v]. The forced oscillation technique offers a non invasive method of assessing lung mechanics that requires only passive patient cooperation [vi,vii]. FOT can also be applied in adults, and is also useful when spirometry is difficult, unpractical or infeasible, for example in the assessment of the elderly, paralysed and unconscious as well as in sleep studies and with mechanically ventilated patients.
The forced oscillation technique was first introduced in 1956 by Dubois and colleagues [viii] as a method of characterizing respiratory mechanics. In this technique, low-amplitude pressure oscillations are applied at the patient's airway opening during spontaneous breathing, the mechanical properties of the patient's respiratory system are derived from the pressure and flow signals recorded at the airway opening. Respiratory system impedance (Zrs) is a ratio of the Fourier transforms of pressure and flow, where the real and imaginary parts of Zrs are the resistance (Rrs) and reactance (Xrs) of the respiratory system. These mechanical properties of the respiratory system are indicative of airway obstruction. FOT has been shown to provide reproducible Rrs values at fixed frequencies as a function of sex, age and height in children [ix]. Mean Rrs has been found to provide reproducible and reliable results in healthy children, asthmatic children and children with cystic fibrosis. These results are concordant with forced expiratory volume in 1 second (FEV1) measures in the children who were able to perform FEV1 [x,xi,xii,xiii,xiv].
FOT studies in adults have shown that mean Rrs and mean respiratory system reactance (Xrs) provide an indicator of airway caliber and can distinguish between asthma, chronic bronchitis and emphysema [xv,xvi,xvii].
Airway hype rresponsiveness is the exaggerated airway narrowing which occurs in response to airway challenge with a wide variety of pharmacological agonists and non-specific irritants such as cold, dry air and oxidant gases [xviii]. One standard measurement of airway hyperresponsiveness is performed by delivering methacholine or histamine to inhibit deep inhalations and increase airway hyperresponsiveness in increasing doses and measuring FEV1 after each dose. Asthma is indicated by a smaller concentration that elicits a measured decrease in FEV1 indicating a greater reactivity of the airways [xix,xx]. More recently, increased variation in airway resistance, standard deviation of Rrs measured by FOT in adults, has been shown to be a useful measure of airway smooth muscle reactivity and thus bronchial hyperresponsiveness [xxi]. Airflow limitation in bronchial hyperresponsiveness is largely determined by airway smooth muscle constriction. Airway diameters have been shown to be constantly changing within a breathing cycle and over short periods of time [xxii]. This leads to a respiratory system resistance that also varies over a breathing cycle, which can be reduced by deep inhalation [xxiii]. Lack of significant bronchodilation or bronchoprotection due to deep inhalation may contribute to the variability in airway calibre that characterizes asthma [xxiv,xxv].